This invention relates generally to electric motors and more particularly to three degree of freedom control of electric planar motors.
Electric motors are used in a variety of electrical equipment. For example, wafer stages utilize linear or planar electric motors to position a wafer during photolithography and other semiconductor processing.
U.S. Pat. No. 5,623,853, entitled xe2x80x9cPrecision Motion Stage with Single Guide Beam and Follower Stagexe2x80x9d to Novak et al. and U.S. Pat. No. 5,528,118, entitled xe2x80x9cGuideless Stage With Isolated Reaction Stagexe2x80x9d to Lee (xe2x80x9cthe ""118 patentxe2x80x9d) discuss examples of semiconductor fabrication equipment and are incorporated herein by reference in their entireties.
U.S. Pat. No. 4,654,571, entitled xe2x80x9cSingle Plane Orthogonally Moveable Drive Systemxe2x80x9d to Hinds (xe2x80x9cthe ""571 patentxe2x80x9d) and U.S. Pat. No. 4,535,278, entitled xe2x80x9cTwo-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatusxe2x80x9d to Asakawa (xe2x80x9cthe ""278 patentxe2x80x9d) discuss two-dimensional planar electric motors. The motors are two-dimensional in that they have two-dimensional arrays of magnets and armatures instead of magnet tracks and one-dimensional armatures. Further, the magnet arrays and two-dimensional armatures may move with respect to each other in more than two dimensions depending upon the design. Conventional two-dimensional linear motors typically have an array of magnets and an armature having one or more coils disposed on one side of the array of magnets.
The ""278 patent describes a three degree of freedom planar motor and a control method for independently producing forces in the X, Y and xcex8z (rotation about Z) directions. The method described in the ""278 patent is better suited for a moving-coil electric planar motor. In a moving magnet array electric planar motor, the ""278 patent describes a method of controlling the motor that can only produce force with groups of four coils that are fully within the magnetic field of the magnet array. In other words, only coils which are completely covered by the magnet array are used to generate forces and coils which are only partially covered by the magnet array are not used to generate forces. Thus, to provide three degree of freedom control of the planar motor at every position, the magnet array must be sufficiently large to cover an area of at least 5 by 3 coils such that 4 by 2 coils are always completely within the magnetic field of the magnet array at all times.
A platform is attached to the two-dimensional electric motor such that it can be moved and positioned in two or more dimensions by the electric motor. For example, a wafer stage in semiconductor processing equipment may be attached to a coil array or magnet array of a two-dimensional motor and the two-dimensional motor would control the positioning of the wafer stage.
When used to position a platform, conventional two-dimensional electric motors do not smoothly and accurately position the platform. Presently, the coil array in a typical two-dimensional electric motor moves with respect to a stationary magnet array. As exemplified in the ""571 patent, cables and hoses are attached to the coil assembly to supply electrical current and coil cooling fluid or air supply to the coils, respectively. However, the hoses and cables impede free motion of the coil assembly.
Thus, it would be desirable to provide an accurate and efficient method of controlling a planar electric motor such that it can be driven to position the platform in three degrees of freedom. It would also be desirable to provide a planar electric motor whose accuracy in positioning is not limited by wiring and/or coil coolant hoses.
The present invention comprises a system and method for controlling electric planar motors to move and position in three degrees of freedom. The electric planar motor of the present invention is preferably a moving magnet array electric motor comprising a magnet array and a coil array. The current supplied through coils of the coil array interacts with the magnetic field of the magnets of the magnet array to generate forces between the magnet and coil arrays. The generated forces provide motion of the magnet array relative to the coil array in a first direction, a second direction, and rotation about a third direction. The present invention teaches the levels of current to be applied to the coils to achieve accurate control of the electric planar motor in three degrees of freedom.
The controlling method of the present invention is achieved by the interaction of current in the coil and a magnetic field associated with the magnet. The method for controlling a planar electric motor for positioning in three degrees of freedom, includes (1) positioning a magnet array over a portion of a coil array, the magnet array including magnets with magnetic fields and the coil array including coils generally disposed in a plane; (2) determining the currents to be applied to the coils to generate forces between the magnet array and the coil array in a first (X) and second (Y) directions defined by the plane; (3) determining a resultant torque between the magnet array and the coil array generated by the forces; (4) determining current adjustments to compensate for or cancel out the resultant torque; and (5) applying a sum of the currents and current adjustments to the coils to interact with the magnetic fields of the magnet array. The method may also be adapted and used to determine the current to be applied to control a planar electric motor in three degrees of freedom.
The method for controlling a planar electric motor for positioning in three degrees of freedom may further include determining the position of the magnet array relative to the coil array and using the determined position in determining currents, resultant torque and/or current adjustments. The currents to be applied to coils in the portion of the coil array may be selected from sinusoidal, triangular and square waveforms. The method may further include determining the forces to be generated between the magnet array and the coil array in the X and Y directions to result in forces in the X and Y directions and/or the torque about Z. The current adjustments may be determined for each coil in the portion of the coil array.
The present invention utilizes all the coils within the magnetic field of the magnet array, including those which are only partially within the magnetic field of the magnet array. Thus, the currents to be applied to the coils may be determined only for coils in that portion of the coil array and the sum of the currents and current adjustments is applied only to coils in that portion of the coil array.
In another aspect of the present invention, a method for positioning a wafer in a lithography system is provided. The method includes: (1) providing a frame, a stage for supporting the wafer and movable to position the wafer relative to the frame, a coil array attached to the frame, and a magnet array adjacent a portion of the coil array, the magnet array being attached to the stage and having magnets generally disposed in a plane, the plane defining a first and second direction; (2) determining currents to be applied to coils in the portion of the coil array to generate forces between the magnet array and the coil array in the first and second directions; (3) determining a resultant torque between the magnet array and the coil array generated by the forces; (4) determining current adjustments to compensate for the resultant torque; and (5) applying a sum of the currents and current adjustments to the coils to interact with magnetic fields of the magnet array.
In another aspect of the present invention, a planar motor is provided. The planar motor includes (1) a first member; (2) a second member that interacts with the first member to generate driving force, the second member being movable relative to the first member at least two directions by the driving force; and (3) a controller connected to at least one of the first member and second member, the controller determining information related to a resultant torque between the first member and the second member.
The planar electric motor and its control method may also be used in a positioning device. The positioning device has a support member such as a stage, a magnet array, and a coil array. In a moving magnet array planar motor, the support member is attached to the moving magnet array and is positionable by the electric motor in three degrees of freedom. Alternatively, the support member is attached to the moving coil array and is positionable by the electric motor in three degrees of freedom.
The method appropriately commutates coils and results in accurate and precise movement and positioning of the stage. The methods provide an electrical current distribution or commutation to the active coils of the coil array to control movement of the magnet array with respect to a coil array.
The method applies to both moving magnet and moving coil electric motors although the moving magnet array electric motor is preferred. The moving magnet array electric motor provides more accurate positioning than conventional moving coil electric motors. The moving magnet electric motor does not require wire connections or cooling hoses to the moving part of the motor. Conventional two-dimensional electrical motors are moving coil motors having wires and cooling hoses connected to the moving coil array. By eliminating wires and hose connections to the moving component, positioning devices using the moving magnet array are more accurate than conventional moving coil platforms. However, although the present invention is described in terms of a moving magnet array electric motor, the electric motor may be modified to be a moving coil array electric motor wherein the coil array moves relative to the magnet array.
The invention""s electric motors and positioning devices should be useful in environments requiring precise and wide ranges of positioning. The electric motor and method of the present invention is particularly useful in positioning wafers in semiconductor fabrication processes.